Add machine-readable pagination using HTML4.01 `<link rel="next">`, etc.

Add machine-readable pagination using HTML4.01 `<link rel="next">`, etc.

These extra `<link>` elements only appear on pages where pagination
makes sense. They trigger functionality in some user agents, such as
Opera's Navigation Bar for more easily navigating forward and backwards
across a paged set of notices, messages, or group lists, etc.